The present invention relates to a multijunction semiconductor device.
A multijunction semiconductor is one having a plurality of pin-junctions and/or pn-junctions. The structure thereof is a multilayered pinpin . . . structure or a pnpnpn . . . structure, and a recombination of carriers occurs at the p/n interface. A multijunction semiconductor of a amorphous silicon is used as a solar cell capable of generating a high-voltage, and also is used as a photo-sensor, rectifier, and the like.
It is considered that properties of such devices are not considerably so deteriorated by light because the thickness of the i-layer on the light-incident side can be relatively small as compared with the thickness of that in a singlejunction photoelectric device. However, a multi-junction semiconductor has a disadvantage in that the Group III atom and/or the Group V atom in the p-layer and n-layer, respectively, are diffused to the other layer through a p/n interface, so that a p/n interface does not function well. Thus, properties of such devices are deteriorated by heat owing to thermal diffusion of dopant atoms. Especially, when an amorphous silicon solar cell is used in the open air, the temperature of the surface of the solar cell reaches about 80.degree. C., so that the properties of the solar cell, e.g. short circuit current, open circuit voltage, fill factor, and the like are remarkably lowered, and as a result, conversion efficiency of the solar cell is decreased. This is because a thermal diffusion easily takes place in a hydrogenated amorphous semiconductor as compared with that of a single crystalline semiconductor.
Besides, a reduction of quality of a multi-junction semiconductor also occurs on a surface between a metal electrode and a semiconductor layer owing to a thermal diffusion of metal to the semiconductor that comes in contact with the electrode, such as disclosed by Takada et al in U.S. patent application No. 744,599 and 744,602 respectively filed on June 14, 1985.
An object of the present invention is to provide a heat-resistant multijunction semiconductor device capable of reducing the decrease in quality of a p-layer and a n-layer owing to diffusion of dopant atoms through a surface of the p/n interface when the multijunction semiconductor device is used at a high temperature.
This and other objects of the invention will become apparent from the description hereinafter.